1. Field of the Invention
The present invention relates to storable water-silica suspensions and methods of cementing wells.
2. Description of the Prior Art
Hydraulic cement compositions are commonly utilized in subterranean well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby strings of pipe such as casings and liners are cemented in well bores. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of a well bore and the exterior surfaces of a pipe string disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe string in the well bore and bonds the exterior surfaces of the pipe to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
When temperatures in the well bore exceed about 230° F., silica has heretofore been included in the cement composition to prevent cement strength retrogression whereby the strength of the set cement composition decreases and the permeability of the set cement increases. As a result of such strength retrogression, pressurized formation fluids can enter the well bore which is highly disadvantageous.
The degradation of Portland and the equivalent set cement compositions first occurs in the range of from about 210° F. to about 230° F. This degradation has heretofore been prevented by the addition of crystalline silica such as fine silica flour to the cement composition which forms tobermorite gel in the cement composition. The tobermorite is stable up to a temperature in the range of from about 300° F. to about 330° F. To maintain the strength of the cement composition above about 300° F., crystalline silicas such as silica flour or a mixture of silica flour and sand must be present to form truscottolite or xontolite gel in the cement which prevents strength retrogression. Thus, fine silica flour, silica flour, sand, and a mixture of silica flour and sand depending upon the temperature encountered by the cement composition have heretofore been used to prevent strength retrogression of set cement compositions.
Suspensions of micro-silica, fine silica flour, silica flour, sand and mixtures thereof in water have been used heretofore in various applications including in well cements to prevent set cement strength retrogression. The water-silica suspensions are convenient for transferring the silica into a cement composition. That is, the water-silica suspensions can be added to a cement composition mixing water thereby avoiding silica dust which if inhaled is detrimental to human health. The water-silica suspensions can be prepared on shore and shipped to well locations in containers or drums. However, the water-silica suspensions used heretofore settle and separate over time, and as a result, agitating equipment during storage of the suspensions must be utilized to prevent such settling and the formation of hard cake in the bottoms of containers.
Thus, there are needs for improved water-silica suspensions which are highly resistant to settling and which after being stored can simply and easily be stirred and poured prior to use.